He started to participate the end of day 2 and he participated day 3 and 4. He also participated in the night meetings. I don't want to encourage this strategy, but I don't find it enough to warrant a vote to OT to ISP either.

As long as we can discourage people from that sort of abhorrent strategy then that's what matters here. if an ISP vio will do that, then I think that's the way to go, but that's unacceptable on day 1 imo.